Brennan is a veteran of more than three decades in intelligence work, and is currently serving as Obama's top counter-terrorism adviser in the White House. Any thought he had of becoming CIA director four years ago vanished amid questions about the role he played at the CIA when the Bush administration approved waterboarding and other forms of "enhanced interrogation" of suspected terrorists.
In a statement at the beginning of Thursday's session, Brennan said the United States remains at war with al-Qaida and other terrorists and is under "daily cyberattack" by foreign countries and others.
He said historic transformations continue sweeping through the Middle East and North Africa, with "major implications for our interests, Israel's security, our Arab partners and the prospects for peace and stability throughout the region." Additionally, he said that Iran and North Korea "remain bent on pursuing nuclear weapons and intercontinental ballistic missile delivery systems."
The hearing was interrupted repeatedly - once before it began and then several times before Brennan had completed his preliminary remarks. At one point, Sen. Dianne Feinstein, D-Calif., the panel's chairman, briefly ordered the proceedings halted and the room cleared so those re-entering could be screened to block obvious protesters.
The shouted protests centered on CIA drone strikes that have killed three American citizens and an unknown number of foreigners overseas.
It was a topic very much on the mind of the committee members who eventually will vote on Brennan's confirmation.
In the hours before the hearing began, President Barack Obama ordered that a classified paper outlining the legal rationale for striking at U.S. citizens abroad be made available for members of the House and Senate intelligence panels to read.
It was an attempt to clear the way for Brennan's approval, given hints from some lawmakers that they might hold up confirmation unless they had access to the material.
White House spokesman Jay Carney said Wednesday the White House is making "extraordinary accommodation" in allowing lawmakers to view classified Justice Department legal advice on drone strikes against Americans. Carney said the White House does not plan to send the Justice memos to lawmakers beyond those on the House and Senate intelligence committees.
Responding to the assurances from the administration, Sen. Ron Wyden, D-Ore., said he was encouraged when Obama called him on the telephone to inform him of his decision.
However, Wyden said that after he went to read the material he became concerned the Department of Justice "is not following through" on the commitment. He asked Brennan to look into the matter, and the CIA nomine said he would.
Despite the sometimes-combative questioning, Brennan's confirmation seemed a foregone conclusion as he appeared before the committee. "I look forward to working with you," said Sen. Mark Udall, D-Colo.
Wyden made the drone strikes the main focus of his time to question Brennan, asking at one point what could be done "so that the American people are brought into this debate and have a full understanding of what rules" are for their use.
Brennan said the day's hearings were part of that effort, and said he backs speeches by officials as a way to explain counter-terrorism programs. He said there is a "misimpression by the American people' who believe drone strikes are aimed at suspects in past attacks. Instead, he said, "we only take such actions as a last resort to save lives" when there is no other alternative in what officials believe is an imminent threat.
Aides have portrayed Brennan as cautious in the use of drones, restraining others at the CIA or military who seek to use them more often. At the same time, as the White House's counterterror adviser, he has presided over an explosion of drone strikes in Pakistan, Yemen and Somalia.
Fewer than 50 strikes took place during the Bush administration, while more than 360 strikes have been launched under Obama, according to the website The Long War Journal, which tracks the operations.